The present invention relates generally to centrifugal compressors and, more particularly, to a centrifugal compressor which provides a particularly wide operating range and a high efficiency.
Generally, in diffusers having stationary vanes, the ratio of the sectional areas between the stationary vanes at the outlet and inlet sides of flow passages is set to a value larger than 1 (the sectional area at the outlet side is greater than the sectional area at the inlet side) in order to reduce the flow rate of a fluid by the stationary vanes. On the other hand, for a reduction in the overall size of a compressor, it is desirable to minimize the outer diameter of a diffuser for the compressor. It is known that it is effective to increase the widths of the stationary vanes at the outlet side relative to the widths thereof at the inlet side in minimizing the outer diameter of the diffuser (as disclosed, for example, in Japanese Unexamined Patent Publication No. 58-183899).
In centrifugal compressors having diffusers with vanes, the operating range is limited by the diffuser; it is limited by the occurrence of choking at the high-flow-rate side and is limited by stall of the diffuser at the low-flow-rate side. The sectional area of the flow passages between the stationary vanes dominantly influences the occurrence of choking, and the sectional area of the flow passages between the stationary vanes and the vane angle influence the stall. As means for preventing the stall, solutions are known in one of which the front edges of the stationary vanes are inclined from the shroud to the main shroud and in the other of which auxiliary vanes are provided in the vicinity of the front edges of the stationary vanes (for example, Japanese Unexamined Patent Publication 1-247798).
In the arrangement disclosed in Japanese Unexamined Patent Publication No. 58-183899, the spacing between stationary vanes is varied so as to be larger at the inlet side than at the outlet side and, accordingly, the vane angle at the outlet side is closer to the radial direction than that at the inlet side so that the flow at the outlet is closer to the radial direction, with a result that the loss is increased at the low-flow-rate side (the loss is particularly large if a scroll is formed on the downstream side of the diffuser). The number of working steps is increased since the surface on which the stationary vanes are formed or supported is curved.
In the arrangement disclosed in Japanese Unexamined Patent Publication 1-247798, it is possible to suppress the stall even at the low-flow-rate side by the effect of the shape of the front edges of the stationary vanes or the auxiliary vanes, but the loss at the downstream side of the diffuser is large.